Re: [PATCH v4 6/7] blk-mq: fix freeze queue race

[Jens Axboe]

On 09/28/2015 08:48 AM, Tejun Heo wrote:
> Hello,
>
> On Sun, Sep 27, 2015 at 10:06:05PM +0900, Akinobu Mita wrote:
> > > >   void blk_mq_finish_init(struct request_queue *q)
> > > >   {
> > > > +     mutex_lock(&q->mq_freeze_lock);
> > > >        percpu_ref_switch_to_percpu(&q->mq_usage_counter);
> > > > +     mutex_unlock(&q->mq_freeze_lock);
> > >
> > > This looks weird to me.  What can it race against at this point?
> >
> > The possible scenario is described in commit log (1. ~ 7.).  In summary,
> > blk_mq_finish_init() and blk_mq_freeze_queue_start() can be executed
> > at the same time, so this is required to serialize the execution of
> > percpu_ref_switch_to_percpu() by blk_mq_finish_init() and
> > percpu_ref_kill() by blk_mq_freeze_queue_start().
>
> Ah, you're right.  I was thinking that percpu_ref_switch_to_percpu()
> being called after blk_mq_freeze_queue_start() would be buggy and thus
> the above can't be enough but that is safe as long as the calls are
> properly synchronized.  Hmmm... maybe we should add synchronization to
> those operations from percpu_ref side.

I think that would be very useful, it seems sort of half-assed if the 
caller side has to provide serialization for that.

--
Jens Axboe

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